1. Field of the Invention
This invention relates to the field of the prevention of reverse engineering of integrated circuits and/or making such reverse engineering so difficult and time-consuming as to make reverse engineering of integrated circuits non-feasible.
More particularly, this invention relates to using, in order to prevent and/or discourage such reverse engineering, apparent metal contact lines terminating on field oxide.
2. Description of the Related Art
The design and development of semiconductor integrated circuits require thorough understanding of complex structures and processes and involve many man-hours of work requiring high skill, costing considerable sums of money.
In order to avoid these expenses, some developers stoop to the contentious practice of reverse engineering, disassembling existing devices manufactured by somebody else, and closely examining them to determine the physical structure of the integrated circuit, followed by copying the device. Thus, by obtaining a planar optical image of the circuits and by studying and copying them, typically required, product development efforts are circumvented.
Such practices harm the true developer of the product and impairs its competitiveness in the market-place, because the developer had to expend significant resources for the development, while the reverse engineer did not have to.
A number of approaches have been used in order to frustrate such reverse engineering attempts, particularly in the field of semiconductor integrated circuits.
For instance, U.S. Pat. No. 5,866,933 to Baukus, et. al. teaches how transistors in a complementary metal oxide-semiconductor (CMOS) circuit can be connected by implanted, hidden and buried lines between the transistors. This hiding is achieved by modifying the p+ and n+ source/drain masks. The implanted interconnections are further used to make a 3-input AND-circuit look substantially the same as a 3-input OR-circuit.
Furthermore, U.S. Pat. Nos. 5,783,846 to Baukus, et. al. and 5,930,663 to Baukus et. al. teach a further modification in the source/drain implant masks, so that the implanted connecting lines between transistors have a gap inserted, the length of which is approximately the length of the feature size of the CMOS technology being used. These gaps are called “channel blocks.”
If the gap is “filled” with one kind of implant (depending on whether the implanted connecting line is p or n), the line conducts; if another kind of implant is used for the gap-filling, the line does not conduct. The reverse engineer must determine connectivity on the basis of resolving the “n” or “p” implant at the minimum feature size of the channel block. In addition, transistor sizes and metal connection routings are modified, in order to deprive the reverse engineer of using clues which can help him find inputs, outputs, gate lines and so on as keys to the circuit functionality.
Practicing the inventions taught in the above-mentioned patents to secure an integrated circuit causes the reverse engineer to perform steps that are not always needed. These steps include: decomposing the circuit layer by layer, careful processing of each layer (which usually must include an etching step) followed by imaging of the layer with exact registration to other layers.
When the reverse engineer is delayering the circuit, he can look also for metal lines running from drain contacts to a poly-gate contact. He does this by looking in the two lowest metal layers for dimples, indicating the presence of metal plugs beneath. Thus, the contact position can be determined, greatly simplifying the reverse engineer's task. Previous patents mentioned above do not address this problem.
Therefore, there still exists a need for an inexpensive, easy-to-implement defensive method which can help to provide the enhanced protection against the reverse engineering of semiconductor integrated circuits, in particular to make the reverse engineer's task of finding real contacts to source and drains very difficult. The present invention provides such a method.